Alternator Load Input Failed Low
Your alternator isn't sending the right load signal to the engine computer, like a speaker that won't tell the amplifier how loud to play. The ECU can't properly regulate charging, which may leave your battery undercharged.
The ECU monitors the alternator load signal (usually a voltage or frequency input) to adjust engine idle and fuel delivery for proper charging. When this signal drops below the expected threshold, the ECU cannot compensate for alternator demand. The fault triggers when the load input remains abnormally low for a set duration.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Alternator Load Signal Voltage | Varies with load (typically 2-5V depending on manufacturer) | Below 0.5V or absent signal |
| Load Input Frequency/PWM Signal | Active switching pattern present | No valid signal detected for >2 seconds |
Code P1250 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P1250 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.